The impact toughness of 0.2% C–1.5% Si–(1.5–5.0)% Mn (mass%) transformation‐induced plasticity (TRIP)‐aided steels with bainitic ferrite and/or martensite structure matrices produced by isothermal transformation process is investigated for automotive body applications. The highest impact toughness, highest Charpy impact absorbed value (CIAV) at 298 K (130 J cm−2), and lowest ductile–brittle transformation temperature (DBTT, 203 K) is achieved in 1.5% Mn steel subjected to an isothermal transformation process at a temperature below the martensite start temperature MS − (50–100) K. An increase in the Mn content results in a small decrease in the CIAV (5–40 J cm−2) and an increase in the DBTT (20 K). The highest CIAV of 1.5% Mn steel compared with steels containing higher Mn content is mainly caused by (i) a more softened matrix structure consisting of wide lath‐martensite and bainitic ferrite, (ii) a smaller amount of the narrow‐lath martensite‐retained austenite‐like phase, and (iii) more stable retained austenite. The lowest DBTT of 1.5% Mn steel compared with the other steel compositions is mainly associated with the low Mn concentration of the matrix in this steel.